TANDEM-X COMMISSIONING PHASE STATUS Jaime Hueso Gonzalez, Markus Bachmann, Harald Hofmann German Aerospace Center (DLR), Oberpfaffenhofen ABSTRACT After the recent launch of the TanDEM-X satellite, the twin of TerraSAR-X, its demanding commissioning phase has started. On the one hand, it has to ensure the same monostatic operation performance as TerraSAR-X. On the other hand, the bistatic aspects have to be verified, which are essential for the acquisition of the global digital elevation model (DEM). This has to be done in a limited time period in order to keep the required nominal operation duration. This paper shows the summary of the commissioning phase activities and its running status. Index Terms—TanDEM-X, TerraSAR-X, bistatic SAR, commissioning phase, formation flying 1. INTRODUCTION TanDEM-X [1] is an Earth observation mission, result of the cooperation of the German Aerospace Center (DLR) and EADS Astrium GmbH, consisting of two twin synthetic aperture radar (SAR) satellites, TerraSAR-X (TSX) and TanDEM-X (TDX). TSX has been operative since 2007, and TDX has been launched on June 2010. Both satellites fly in a versatile, configurable and closely controlled helix orbit formation in order to acquire single-pass cross-track interferometric SAR (InSAR) images of the Earth with the optimal performance. The goal is to generate a global highly accurate digital elevation model (DEM) with 12 m resolution, 10 m absolute- and 2 m relative-height accuracy. This has to be achieved over the whole Earth’s land surface before July 2014. In order to assure the accurate completion of this DEM product, TDX has to undertake a precise commissioning phase in a tight time schedule. The commissioning phase activities range from the monostatic calibration (analogue to the one performed for TSX) to the commissioning of bistatic aspects like the innovative flight formation, the collision avoidance strategy, the upgraded mission planning and ground station network, the synchronization performance, the baseline calibration and the illumination risk handling. The paper describes the status of this challenging commissioning phase and a detailed description of the main phases and activities. 2. CHALLENGES 2.1. Mission Schedule The TSX satellite was launched in June 2007. Since its successful commissioning phase, where the instrument was accurately calibrated, TSX has remained very stable and reliable [2]. However, the warranted life time is limited. For acquiring the TanDEM-X DEM with the specified quality, at least two to three years of combined operation are required. Difficult terrain like mountainous areas needs to be acquired three or even four times to minimize shadowing and layover effects and phase unwrapping errors. In order to start as soon as possible with the nominal DEM acquisition, the TDX commissioning phase has to be completed in due time. 2.2. Complexity The combination of the TerraSAR-X and TanDEM-X mission objectives increases the complexity of the system. Apart from acquiring the InSAR images for the DEM acquisition, the TerraSAR-X customers should not be disturbed. This is only possible by implementing a new mission planning concept to distribute the TerraSAR-X mission acquisition equally on both satellites. The SAR processing chain has also been extended to include an additional branch for interferometric processing [3], comprising bistatic and pursuit monostatic operation modes. The duration of the maintenance phases for activating the TanDEM-X functionalities have to be minimized. 2.3. Collision Avoidance For the first time two satellites are operated at a nominal distance of a few hundred meters only. Safety is ensured by flying in a so-called “helix formation”, where TSX and TDX have slightly different orbital planes [4]. 2.4. Exclusion Zones The helix formation results in one satellite turning around the other with one orbit periodicity. This means that there is a risk of mutual illumination during radar activities, when one satellite is in the direction of the main beam of the neighbor SAR antenna, which could cause damage to the electronic equipment. Therefore, a three-step fail-safe concept has been implemented to disable the transmission during this risky illumination periods.